For a transition circuit between a waveguide and a microwave transmission line formed on a dielectric substrate, there is a transition circuit as described in JP-A-6-140816, for example. With this transition circuit, a constructional example of a waveguide and a microstripline is shown.
In the conventional device described in the above literature, the dielectric substrate on which an open stub is formed is inserted into the waveguide through a notched portion formed by removing a portion of the side of the waveguide. At that time, the dielectric substrate is inserted thereinto such that a hollow is formed between the lower side of the dielectric substrate and the end face of the waveguide, thereby constructing the transition circuit.
In other words, when the microwave is inputted through the waveguide, the microwave is reflected by the end face of the hollow formed beneath the dielectric substrate. The phase of the reflected microwave has a phase sift of 180 degrees to the phase of the inputted microwave. For this reason, both the microwaves become in phase with each other at the location approximately ¼ wavelength away from the end face of the hollow in a direction axially of the tube of the waveguide, and strengthen each other by interference.
Therefore, the dielectric substrate on which the open stub is formed is inserted at the position approximately ¼ wavelength away from the end face of the hollow in a direction axially of the tube of the waveguide.
Thereby, the microwave transmitted from the open-stub side of the dielectric substrate placed within the waveguide, is transmitted to a conductor line portion, which is exposed outside the waveguide and is connected to this open stub, through the notched portion of the waveguide.
Consequently, the conductor line portion positioned outside the waveguide and connected to the open stub functions as a microwave transmission line, resulting in the transition of the transmission path of the inputted wave from the waveguide to the microwave transmission line formed on the dielectric substrate.
In this connection, the insertion of the dielectric substrate thereinto actually makes the position at which the incident wave and the reflected wave become in phase with each other to be deviated from the above-mentioned position; however, the proper adjustment for the position where both the waves become in phase makes the above-described device to operate as the transition circuit.
In the conventional transition circuit, there is a problem that the hollow is formed beneath the dielectric substrate projecting through the notched portion into the waveguide, thereby increasing the thickness in the circuit construction.
Moreover, there is a problem that if a multilayer dielectric substrate is used, wiring can not be carried out at all in the portion thereof which is inserted into the waveguide.
Further, there is a problem that when a through hole is made through the dielectric substrate and further, the notched portion of the waveguide sandwiches the substrate between the upper part and the lower part of the portion in order to construct the transition circuit, the occurrence of a misalignment between the positions of the upper and lower inner walls of the waveguide deteriorates the performance of the transition circuit itself.
The present invention has been accomplished to solve the above-mentioned problem. An object of the present invention is to provide a transition circuit the thickness of which can be reduced without the need for providing a particular hollow under the dielectric substrate. Furthermore, an object of the present invention is to provide a transition circuit in which a high-frequency line and lines for the power supply and the control signal can be wired in the lower layer of the dielectric substrate when a dielectric multilayer substrate is used.